The Swedish start-up Minesto is working on a new kind of underwater turbine, called "Deep Green." Here's how it works: You tether this kite-like apparatus to the ocean floor, at a depth of anywhere from 60 to 150 meters. Then, as a tide or current pushes against the kite, it moves from side to side because its wings create a lift force. As it moves, water flows through a turbine in the kite. And because it's moving, the velocity of the flow of water through the turbine can be 10 times the surrounding stream flow, according to Minesto. Here's an animation.
In theory, Deep Green should be much cheaper to install and maintain than other wave-power technologies, beause these things are relatively small. Each kite's wingspan is about 40 feet. And tidal power is attractive because it's predictable. It should produce 500 kilowatts of power when it's working. To put that into perspective, the average American home uses about 11,040 kilowatt-hours each year. So one of these kites, working for an hour, could supply about two weeks worth of power for the average home.
Minesto just raised more than $2.5 million to run a trial off the coast of Northern Ireland in 2011 (pdf). They're hoping to make Deep Green commercially available within four years.
And if they don't work, at least they won't destroy our coast when they break.